Sum of consecutive numbers in JavaScript

Let's say, we have to write a function that takes in an array and returns another array in which the consecutive similar numbers are added up together.

For example ?

const array = [1, 5, 5, 5, 8, 8, 9, 1, 4, 4, 2];
console.log("Original array:", array);

Original array: [ 1, 5, 5, 5, 8, 8, 9, 1, 4, 4, 2 ]

The output should be ?

[1, 15, 16, 9, 1, 8, 2]

All consecutive 5s added up to 15, then 2 consecutive 8s added up to 16 similarly 4s added up to 8.

Using Array.reduce() Method

We will use the Array.prototype.reduce() method to reduce the original array and simultaneously construct a new one.

const array = [1, 5, 5, 5, 8, 8, 9, 1, 4, 4, 2];

const sumConsecutive = (array) => {
    return array.reduce((acc, val) => {
        if (acc.last === val) {
            acc.arr[acc.arr.length - 1] += val;
        } else {
            acc.arr.push(val);
            acc.last = val;
        }
        return acc;
    }, { arr: [], last: undefined }).arr;
};

console.log(sumConsecutive(array));

[ 1, 15, 16, 9, 1, 8, 2 ]

How It Works

The function uses an accumulator object with two properties:

• arr: Stores the result array
• last: Tracks the previous element

For each element, if it matches the last seen value, we add it to the most recent sum. Otherwise, we start a new group.

Alternative Approach Using For Loop

const sumConsecutiveLoop = (array) => {
    if (array.length === 0) return [];
    
    const result = [];
    let currentSum = array[0];
    
    for (let i = 1; i < array.length; i++) {
        if (array[i] === array[i - 1]) {
            currentSum += array[i];
        } else {
            result.push(currentSum);
            currentSum = array[i];
        }
    }
    
    result.push(currentSum); // Add the last sum
    return result;
};

const testArray = [1, 5, 5, 5, 8, 8, 9, 1, 4, 4, 2];
console.log(sumConsecutiveLoop(testArray));

[ 1, 15, 16, 9, 1, 8, 2 ]

Conclusion

Both approaches effectively sum consecutive duplicate numbers. The reduce method is more functional, while the for loop approach is more straightforward and potentially easier to understand.